High rate packet data (HRPD) is also commonly referred to as 1xEV-DO which is a high-speed code division multiple access (CDMA) based wireless data technology. A comprehensive discussion of HRPD can be found in the Telecommunications Industries Association's interim standard TIA/EIA/1S-856 entitled cdma2000 High Rate Packet Data Air Interface Specification, November 2000, which is incorporated herein by reference.
Referring now to FIG. 1, there is shown a diagram of a prior art Orthogonal Frequency Division Multiplexing (OFDM) based enhanced broadcast multicast (EBM) mode for an HRPD proposal (found in proposal entitled “Enhanced Broadcast-Multicast for HRPD”, Philadelphia meeting of 3GPP2, June, 2004, C30-20040607-060). As shown in FIG. 1, the structure of the transmission format 100 employs both OFDM and Code Division Multiple Access (CDMA) transmissions. Medium Access Control (MAC) portions 104, 108, 114, 116 and pilot portions 106 and 116 are transmitted using CDMA, while OFDM data portions 102, 110 and 120 are transmitted using OFDM. One limitation of the transmission format 100 is that it is not backward compatible with the current HRPD transmission scheme 200 shown in FIG. 2. Since OFDM is not currently employed by HRPD channels, the HRPD channels are not backward compatible with the OFDM modulated portions of transmission format 100.
The broadcast/multicast data is broadcast to all system users and it is also possible the same information is also transmitted from different cells/sectors in a communication system employing the format shown in FIG. 1. In this situation, when CDMA with a long code spreading is used, the exact same signals received from the different cells/sectors cause interference with each other and they can be coherently combined with each other using for example a rake receiver. However, the transmission of HRPD requires equalizers to be implemented at the receivers and in this particular case; it is difficult to efficiently combine the same data transmitted by the different cells/sectors in the system. The combining cannot be done efficiently in one operation since data coming from different cells/sectors are modulated with different cell/sector-dependent specific spreading codes. That is, a possible combining scheme performs separate equalization on the received signal from each sector prior to combining the resulting soft decisions from all the equalizers. While this combining approach is feasible, the performance loss due to sub-optimal combining and equalization may be significant especially in high frequency selective channels.
In highly frequency selective channels (example, a Pedestrian B channel), an equalizer can be employed as a substitute to a rake receiver in order to enhance performance. In this case, equalization is performed per sector prior to soft combining across the sectors. Most equalizers require a sufficiently high signal-to-noise ratio (SNR) training to achieve good performance in order to train its equalizer taps or perform channel estimation. Unfortunately, this is not possible since the SNR for the secondary sectors is very low due to interference from the stronger (primary) sectors. Also, due to sector-dependent data and pilot spreading it is not possible to achieve a one shot equalization of the total channel summed across all the sectors.
In the case of OFDM, since the exact same information is transmitted from the different cells/sectors (without the long code), the signals received from the different cells/sectors look like multi-path propagation delay. Hence, all the different multi-paths/signals can be efficiently combined at the receiver using fast Fourier transform (FFT) prior to performing frequency-domain equalization. This approach typically uses a cyclic prefix (CP) having a length that is larger than the expected propagation delay from the different base stations in the system. Hence, a very large CP (e.g., 80 samples requiring approximately 65 microseconds) is in employed with the transmission format of FIG. 1, even though the channel spread for a single base station is expected to be much less (e.g., less than 15 microseconds in a Vehicular B channel). Some loss in spectral efficiency due to cyclic prefix insertion occurs. But this rate loss is offset with the throughput gain from the use of combining.
As shown in FIG. 2, the current transmission format for HRPD which is transmitted using CDMA includes data (traffic data) 202, a MAC, Pilot 206, MAC 208, followed by data 210, data 212, MAC 214, Pilot 216, MAC 218 and data 220. While OFDM allows a very efficient method of joint combining and equalization of the exact replica of the signal transmitted from the different base stations, it also introduces limitations, such as non-backward compatibility with the all CDMA transmission format currently used for HRPD shown in FIG. 2.
The current broadcast/multicast system (BCMS) applies sector-dependent long code spreading on the pilot and data fields which generates a few problems. One problem